8) How many plants/what size plants are needed in a home to achieve air cleaning benefits?

The amount of leaf surface area influences the rate of air purification by plants. Generally, the larger the plant leaf surface area, the higher the transpiration rate and the greater the surface area to absorb airborne chemicals.

Without expensive testing to measure the number and levels of chemical concentrations, it is impossible to say specifically how many plants are needed. As a general rule, I recommend at least two good-sized plants per 100 square feet of space within a home. Of course, if the homeowner can maintain more plants, then it is even better.

For those requiring a more technical answer, I offer the following:

The basis for recommending the number of plants per room is based upon the average amount of air pollution found in public buildings that were tested by the U.S. Environmental Protection Agency (EPA). The EPA monitored the indoor air quality in hospitals, nursing homes and office buildings for several years and published their findings. This data can be found in an EPA report on Indoor Air Quality in Public Buildings: Vols. I and II, August 1988, EPA/00/6-88/009ab.

Data was extracted from this report to produce the following table:

A 100 ft2 (9.3 m2) office with an 8 ft ceiling contains a volume of 800 ft3 (22,640 liters) of air. Based upon the above data: if the air contained 0.173 µg/liter of formaldehyde, then the room would contain a total of approximately 3,917 µg of formaldehyde. Go to the table below and estimate the number of plants required to remove this amount of formaldehyde. Formaldehyde is the predominant chemical found in the test buildings. Therefore, if sufficient numbers of plants are added to remove formaldehyde, removal of other chemicals should occur as well.

* Removal rate may vary with plant size and growth medium.

Note: The figures above are the results of sealed-chamber studies and not ‘real world’ conditions. In a real environment, conditions could vary significantly. Therefore, we recommend that one should at least double the number of plants required based upon this information whenever possible.

Yes. All houseplants give off oxygen and take in carbon dioxide during the photosynthetic process. Many studies have shown that plants also remove airborne chemicals from the indoor environment, including formaldehyde and carbon monoxide. Some plants are more effective at removing chemicals than others.

2) Which houseplants are the best at purifying indoor air? (Can you list the top 10-15 plants in your opinion?)

In my book, ‘How to Grow Fresh Air – 50 Houseplants That Purify Your Home or Office,’ (Penguin, 1997), I rate 50 plants using the following criteria:

(1) Plant leaves can absorb certain organic chemicals and destroy them through a process called ‘metabolic breakdown.’ This was proven by a group of German scientists who labeled formaldehyde with a radioactive carbon 14 tag and followed its absorption and metabolic destruction inside a spider plant. The formaldehyde was metabolized and converted into tissue products such as organic acids, sugars and amino acids as demonstrated by the radioactive carbon 14 label. This information was published in the following peer-reviewed journal. [Martina Giese, Ulrike Bauer-Doranth, C. Langebartels, and Henrich Sanderman, Jr. ‘Detoxification of formaldehyde by the spider plant (Chlorophytum comosum).’ Plant Physiology, 1994, 104: 1301-1309.

(2) When plants transpire water vapor from their leaves, they pull air down around their roots. This supplies their root microbes with oxygen. The root microbes also use other airborne substances, such as volatile organic chemicals, as a source of food and energy. Naturally-occurring microbes living in the root zone (rhizosphere) can rapidly adapt to a chemical contaminant by producing new colonies that are resistant to the chemical. As a result, they become more effective the longer they are exposed to the chemical. It is also important to remember that the efficiency of plants or a filtering device decreases as the concentration of chemicals in the air decreases. For example, the removal rate of a chemical is much higher at 7 parts per million (ppm) exposure than at 2 ppm.

4) Is there anything that these particular plants have in common?

The most effective plants in removing airborne chemicals all have high transpiration rates, meaning they move air more rapidly down to the root zone.

5) Which toxins and gases can these plants help remove from the air?

The Environmental Protection Agency (EPA) has identified as many as 900 volatile organic chemicals (VOCs) in the indoor environment. Many of the chemicals are similar in structure. We studied the chemicals that have been most commonly identified. These chemicals include formaldehyde, benzene, xylene, toluene, ammonia, trichloroethylene, acetone, carbon monoxide, methyl alcohol and ethyl acetate.

6) Is there evidence that more of these toxins and gases build up in the home during winter months when the house is closed up?

A tightly sealed home is more likely to have a buildup of toxins regardless of the season. During winter months, heating of the home dries the air and makes one more susceptible to respiratory ailments. Plants add humidity to the air as they transpire.

7) Do temperature and humidity influence the ability of plants to remove airborne chemicals and microorganisms?

Yes. The transpiration rates of plants are important in removing both airborne chemicals and microbes. When plants transpire (emit) water vapor from their leaves into the air, they also pull air down to their roots. Any airborne contaminants are also pulled down into the plant root zone. Microbes, living on and around plant roots in an area called the rhizosphere, breakdown and destroy the chemicals. Microbes convert these chemicals into a source of food and energy for the plant and themselves.

Both temperature and humidity influence the transpiration rates of plants. Test results show that plants with high transpiration rates are more effective in removing pollutants from the indoor environment. For example, we conducted studies in a home during the winter months using an Areca palm (Chrysalidocarpus lutescens with a height of 56-inches) that was growing in a pot 14-inches in diameter. The test data is shown below.

You want to place plants in the home that the homeowner can maintain, paying attention to the basic needs of light, water and nutrients. No ‘special’ care is needed. However, our studies have shown that plants grown in hydroculture (soil-free) are in general about 25 percent more effective in removing VOCs than plants grown in commercial potting soil. Air flows more readily through hydroculture pebbles to the root zone than through compacted soil. Hydroculture has another added benefit as water and nutrients are introduced through a watering port that takes the water to the root zone. In this method, the pebble surface remains dry at all times and so mold is unable to grow as often happens with soil-grown plants. Hydroculture uses a sealed container, eliminating the drain tray which can often serve as another harbinger of mold spores.

10) What are negative ions?

Ions are charges particles in the air that are formed when enough energy acts on a molecule, such as water, to eject an electron. The displaced electron attaches itself to a nearby molecule that becomes a negative ion.

In nature, ions are formed in a variety of ways, such as UV light, airflow friction, lightning, falling water and by plants. Plant leaves produce negative ions as they emit water vapors. Therefore, plants that have the highest transpiration rates produce the most negative ions. Waterfalls and tropical forests create copious amounts of negative ions. Synthetic building materials, clothing and furniture coverings remove large numbers of negative ions from the indoor environment. The positive static charge of plastics also consumes large quantities of negative ions. Therefore, the negative ion count in modern buildings is often very low.

Since high levels of negative ions are needed for good health, large numbers of interior plants can improve our health and feeling of well-being. Our studies have shown that large numbers of interior plants can reduce the levels of airborne microbes. Although we did not measure negative ion levels, the reduction in mold spores and bacteria in the ambient air was most likely due to negative ions. Our studies were published in 1996 in the Journal of the Mississippi Academy of Sciences {41(2):99-105}.

Dr. Virginia Lohr at Washington State University also published a paper on how interior plants can reduce human stress and increase productivity. These effects are most likely due to increased negative ion levels in offices. {‘Interior plants may improve worker productivity and reduce stress in windowless environments,’ Journal of Environmental Horticulture, 1996, 14(2):97-100.}

The positive health effects of negative ions have been known for almost a hundred years. The fact that houseplants produce negative ions is a well-established fact.

11) Do houseplants increase dust and mold spores in rooms?

No. In fact, foliage plants reduce airborne microbes in the ambient air provided that the soil is not exposed. A two-inch (more or less) layer of gravel or other porous material on top of the soil will prevent mold growth. Our studies have shown that plant-filled rooms have 50 to 60 percent less airborne microbes than similar rooms without plants. {B. C. Wolverton and J. D. Wolverton. ‘Interior plants: their influence on airborne microbes inside energy-efficient buildings.’ Journal of the Mississippi Academy of Sciences, 1996, 41(3):99-105}

Therefore, scientific research demonstrates that interior plants can reduce the levels of airborne microbes and dust in rooms whenever sufficient numbers of plants are present. Some allergy physicians continue to recommend their patients remove all plants from their homes. However, there is no scientific basis for this recommendation. In fact, plants should prove beneficial to allergy patients provided the plants are grown in a manner to prevent mold growth on the soil surface.

12) Will houseplants in a bedroom remove the oxygen at night?

No. Do the animals underneath the dense, tropical canopy of the rain forest die at night from lack of oxygen? I don’t think so. It is true that some plants use a small amount of oxygen at night. However, others such as succulents, orchids and bromeliads actually add oxygen to the air at night. The Sanseviera (snake plant or mother-in-law tongue), a common and easy to grow plant, also gives off oxygen at night. The effect one should receive from filling a bedroom with plants is a feeling of breathing health indoor air. The only caveat is to make sure the soil’s surface is covered to prevent the growth of mold spores.

13) Which plants are best for a child’s bedroom?

Some houseplants that are considered safe to have around small children include the following: (*However, it is best to not leave a child unattended around plants.)